Method for growing crystals



Feb. 10, 1970 c. w. HANKS ETAL 3,494,304

. METHOD FOR GROWING CRYSTALS Filed July 15, 1968 3 Sheets-Sheet 1 FIG.I.

CO NTROL CONTROL CIRCUIT CIRCUIT INVENTORS CHARLES W. HANKS BYJHARLES dAHUNT ATTORNEYS Feb. 10, 1970 c. w. HANKS ETAL 3,494,804

I METHOD FOR GROWING CRYSTALS Filed July 15, 1968 3 Sheets-Sheet 2 FIG.3.

a lilllllllll :2 25 /m I NVEN TOR-S CHARLES W. HANKS D BY CHARLES d'AHUNT 3 MVM) ATTORNGN c. w. HANKS ETAL 3,494,804

METHOD- FOR GROWING CRYSTALS Feb. 10,. 1970 3 Sheets-Sheet 3 Filed July15, 1968 CONTROL CIRCUIT CONTROL CIRCUIT FIG. 9.

INVENTORS CHARLES: W. HANK8 CHARLES d'A HUNT BY .fiwfi United StatesPatent US. Cl. 1481.6 11 Claims ABSTRACT OF THE DISCLOSURE A method isdescribed for growing a single crystal from a molten pool formed ofcrystalline material, such as silicon and super alloy metals. Growth ofthe single crystal is controlled by controlling the thermal pattern, inthe molten pool from which the product is grown, through variation inthe position of electron beams directed at the pool surface.

This invention relates to single crystals and, more particularly, to animproved method for growing such crystals from a molten pool, and is acontinuation-in-part of application Ser. No. 659,175, now abandoned.

Certain articles of manufacture utilize material produced in singlecrystal form. For example, many types of electronic circuit elements,such as transistors and diodes, are manufactured from thin slices ofsemi-conductor material produced in single crystal form. In addition,some items which are subjected to high stresses and high temperatures,such as turbine blades, may be constructed of certain super alloy metalsin single crystal form.

One general technique which has heretofore been developed for producingsingle crystals, from which various items mentioned above may bemanufactured, involves the drawing or growing of a generally cylindricalsingle crystal from a molten pool. In the growing technique, a singlecrystal seed is dipped into a molten pool so that an interface is formedbetween the seed and the molten pool. The seed is then withdrawn fromthe pool in a manner which causes the molten material at the interfaceto solidify continuously as the seed is drawn upwardly. The precise wayin which this is accomplished may vary considerably, frequentlydepending on environmental conditions and other factors.

Although satisfactory in some respects, heretofore known ways ofaccomplishing crystal growing often tend to be expensive and diflicultto carry out. Coating flakes, condensate and dirt on the seed surface,etc., can cause nucleation and growth of other crystals, destroying thesingle crystal nature of the final product. Disturbing conditions at theoriginal surface of the seed may result in poor surface quality in thecrystal being grown. Surface imperfections, and surface nucleationagents can be caused to grow out of the surface by reducing the diameterof the crystal at an angle of fresh surface which is greater than about30 from the crystal axis. T 0 do this may be difficult, however, for thereason that it is usually necessary to precisely vary the temperature atthe interface and, in most known systems, temperature has a very slowresponse.

Other difficulties may be encountered during crystal growing. Variationin environmental conditions may have 3,494,804 Patented Feb. 10, 1970 adeleterious effect on quality of the single crystal by producingnon-symmetrical intracrystalline growth and consequent dislocations.Difficulties in achieving a desired crystal size, and diameter, and theintroduction of wanted and unwanted impurities, may also presentproblems in heretofore known ways of crystal growing.

Accordingly, it is an object of this invention to provide an improvedmethod for producing a single crystal.

Another object of the invention is to provide a method for producing asingle crystal by which a high degree of consistency and quality inresults may be obtained.

It is another object of the invention to .provide a method for growing asingle crystal from a seed, which minimizes the effect of crystalnucleation agents that may be residing on the surface of the seed, andwhich allows the reproduction of the subsurface quality of the seed.

A further object of the invention is to provide a method for growing asingle crystal wherein the cross sectional size of the single crystalduring growth may be readily varied.

Other objects of the invention will become apparent from the followingdescription taken in connection with the accompanying drawings wherein:

FIGURE 1 is a schematic sectional view of apparatus for practising themethod of the invention;

FIGURE \2 is a top plan schematic view of part of the apparatus ofFIGURE 1;

FIGURES 3 through 7 are schematic side elevational views illustratingsuccessive steps of one specific way of practicing the method of theinvention;

FIGURE 8 is a schematic perspective view of an alternative type ofapparatus for practicing the invention; and

FIGURE 9 is a schematic full section view of a further alternative typeof apparatus for practicing the invention.

Very generally, the invention operates to grow a single crystal 11 fromcrystalline material formed into a molten pool 12. The pool is heated bybombarding its surface with at least one electron beam 13. A singlecrystal seed 25 is dipped into and drawn from the molten pool to formthe single crystal and the cross sectional size of the single crystalbeing grown is controlled by controlling the position of the areas 14 ofbeam impact on the pool surface, relative to the single crystal.

Referring now more particularly to FIGURES 1 and 2, the apparatus usedin practicing the method of the invention will be described in detail.The apparatus illustrated in FIGURES 1 and 2 is for growing a singlesilicon crystal of generally circular cross section, such as is used inthe manufacture of certain types of transistors and diodes. The singlesilicon crystal 11 is produced in an electron beam furnace having avacuum tight enclosure 16. The region inside the enclosure is evacuated,through a duct 17 in a wall of the enclosure, by a suitable vacuum pump,not illustrated. The pressure inside the enclosure 16 is preferablyreduced to less than one torr.

A crucible 18, preferably comprised of stainless steel, is supportedwithin the enclosure 16. The crucible contains the molten pool 12 fromwhich the single silicon crystal 11 is grown. Coolant passages 19 areprovided in the walls of the crucible 1 8, and a suitable coolant, suchas water, is circulated therethrough. Accordingly, a skull 21 ofsolidified silicon forms between the walls of the crucible 18 and themolten pool 12. This skull prevents any interaction between the cruciblematerial and the molten silicon, and it insures that the molten siliconwill have a high degree of purity.

The molten pool 12 is heated by bombarding its surface with threeelectron beams 13. The electron beams are produced in three electronbeam guns 22, respectively. Only one of the electron beam guns 22 isillustrated in FIGURE 1 in order to present a less confused appear ance.The three electron beam guns 22 illustrated in FIG- URE 2 are shown inblock form. In the illustrated apparatus, the beams are shaped or sweptto produce an almost continuous ring-shaped or annular impact area asshown in FIGURE 2. The inner diameter of the annular impact area isgreater than that of the crystal being pulled, and the outer diameter isless than that of the pool. Preferably, the difference in outer andinner diameters is kept as small as possible consistent with providingsufficient energy to the pool.

The electron beam guns 22 may be of any type known in the art by whichthe beam position may be varied With respect to the single crystal 11,however, the details of one type of electron gun which may be used areillustrated in FIGURE 1. The electron guns 22 are disposed within theenclosure 16 and each includes a directly heated cathode 23 and anaccelerating anode 24. The electrons emitted by the cathode 23 aredirected into a beam by a shaping electrode 26 and the beam thus formedis accelerated by the accelerating anode 24. To this end, the cathode 23and the shaping electrode 26 are maintained at a negative potential withrespect to the anode 24. A transverse field, established by anappropriately positioned electromagnet 27, is used to deflect theelectron beam onto the surface of the material in the molten pool 12.Suitable means, not shown, are provided for heating the cathode and formaintaining the desired potentials on the described elements.

The single crystal 11 is formed by drawing it upwardly out of the moltenpool 12.. The growth of the crystal is initiated by immersing a suitablesingle crystal seed in the pool and beginning a slow upward withdrawal.The seed is held in a suitable clamp 28, and an actuating rod 29 issecured to the clamp 28. A motor driven mechanism, not illustrated, isused to rotate the rod 29' and hence the single crystal 11 while it isbeing drawn upwardly. The crucible 18 may also be rotated in theopposite direction by suitable means, not illustrated. A meniscus 30forms between the single crystal 11 and the surface of the molten pool12 during the growing operation.

When the single crystal 11 is being drawn upwardly, the interface '31,between the solid crystal and the molten pool 12, assumes a generallyspherical shape. As the crystal is drawn upwardly, the molten siliconfreedes at the interface with the desired monocrystalline structure. Asignificant factor which affects the manner in which the siliconsolidifies is the distribution of temperature in the molten materialnear the interface. Various techniques are known in the art forregulating the temperature distribution by regulating the temperature ofthe molten pool. Previous techniques, however, have often been hamperedby a slow response time insofar as regulation of pool temperature isconcerned. This makes it much more diflicult to achieve satisfactorymonocrystalline growth.

In practicing the method of the invention, the average temperature ofthe molten pool 12 is maintained somewhat above the melting point of thesilicon. The temperature in and near the impact areas of the beams,however, is substantially higher than the average temperature of thepool. As a result, the electron beam 13 causes a region of turbulenceadjacent its surface impact area because of the localized production ofheat energy at the surface. This turbulence is characterized by anoutward flow of superheated molten material at and near the pool surfacefrom the regiion of highest heat toward regions of lower heat. A returnflow of cooler material inwardly and then upwardly occurs at a lowerdepth in the molten pool. The flow of turbulence is indicated by thearrows 32 in FIGURE 1, and this turbulence region is generally annular,extending around the crystal being pulled in accordance with the shapeof the corresponding beam impact pattern. The velocity of the moltenmaterial in the turbulence region generally decreases with increasingdistance from the beam impact area.

When the electron beam 13 is moved closer to the crystal 11, thevelocity and temperature of the flow of molten material adjacent theinterface 31 is correspondingly increased. This causes a reduction inthe diameter of the crystal being pulled due to the increasedtemperature and washing action of the superheated molten material.Conversely, when the beam 13 is moved away from the crystal 11, thevelocity and temperature of the flow of molten material at the interface31 is reduced, resulting in an increase in the diameter of the crystalbeing pulled. By regulating the position of the region of turbulencerepresented =by the arrows 32 (i.e., by moving the beam toward or awayfrom the crystal 11) the flow of molten material at the interface 31 maybe regulated as to velocity and temperature to achieve a desired crystaldiameter.

The position of the beams as they are used in practicing the method ofthe invention is dependent upon several factors. Among these factors isthe average pool temperature, the power of the electron beams, the rateat which the crystal is pulled, the size of the impact areas of theelectron beams, and the rate at which heat is removed through the cooledcrucible walls. The precise conditions required for successful operationare established empirically and may usually be determined after a fewtest runs. Some examples of satisfactory operating conditions aredescribed subsequently.

Operation of the illustrated apparatus in accordance With the method ofthe invention, as above described, enables variation and control overthe diameter of the crystal being grown by means which provide a veryfast response. Such a fast temperature response is of significance incrystal growing. For example, variation in diameter of the crystal beingpulled or grown may be accomplished almost immediately after movement ofthe electron beams in the described manner. This facilitates the growingout of crystal nucleation agents and surface imperfections, as describedsubsequently. Moreover, variation in environmental conditions may bevery quickly compensated for by appropriate movement of the beam orbeams. Care must be taken to avoid moving the beams too far away fromthe crystal being grown because, if the temperature of the molten pooldrops sufliciently, a bridge of solid material may form between thecrystal and the central hump in the skull 21.

By using the position of the beams 13 as a primary and rapid means oftemperature control, and by using control over the power level of thebeams as a secondary and slowly changing variable, control over thecrystal diameter may be achieved concurrent with the maintenance ofconstant molten pool conditions. Movement of the impact areas 14 of thebeam results in a changing of the thermal pattern in the pool, therebyeffecting the changes in growth conditions of the crystal as previouslydescribed. It has been found that the ratio of change in the distance ofthe beam impact areas 14 from the axis of the crystal to the change incrystal diameter caused thereby is about 3 or 4 to 1.

At the same time, the size and average temperature of the molten poolmay be kept relatively constant, despite beam impact position changesand heat balance changes, by adjusting the total beam power level (i.e.,adjusting beam current or emitter temperature or both). The constantconditions of purity, temperature, pool size, etc., result in productionof a crystal which is symmetrical in its intracrystalline structure,being substantially free of dislocations, providing the subsurfacestructure of the seed crystal is also substantially free ofdislocations.

In order to replenish the molten pool 12 for silicon removed therefromduring the growing of the single crystal 11, silicon is fed into thepool. In the illustrated embodiment, the feed silicon is in the form ofa solid bar 33 in a commercially obtainable polycrystalline form. Thebar 33 of feed silicon is held in a suitable clamp 34 and is loweredtoward the molten pool by a rod 35 connected to the clamp. A suitablemechanism, not illustrated, is used to lower the rod 35 and to rotateit, thus rotating the silicon bar 33 as it is being lowered toward thepool.

The lower end of the feed silicon bar 33 is melted by means of anelectron beam 36. The electron beam 36 is produced by an electron beamgun 37 of a construction generally identical to that of the guns 22.Accordingly, the various parts of the electron beam gun 37 illustratedin FIGURE 1 have been given reference characters identical with thecorresponding parts in the electron beam gun 22 illustrated in the samefigure. As may be seen in the drawing, the gun 37 is mounted in theenclosure in an attitude which is inverted with respect to the gun 22.This is for the purpose of achieving a desired electron beam direction,as indicated. As has been described, the thermal pattern in the pool 12has a substantial effect on the growth of the single crystal 11.Accordingly, it is desirable to minimize the effect, on the thermalpattern, of the material being fed into the molten pool. Theminimization of the effect of additional material is accomplished bydirecting the electron beam 36 such that it does not strike the surfaceof the molten pool 12, thereby avoiding any thermal imbalance. Controlover the power and direction of the electron beam 36 is provided bymeans of a suitable control circuit 38, illustrated in FIGURE 2. Thelower end of the feed bar 33 is melted by the electron beam 36 into atapering shape and the bar is positioned sufficiently close to thesurface of the molten pool 12 that a meniscus 39 forms between the lowertip of the feed bar 32 and the molten pool. The fee-d rate of thematerial being added is selected such that the surface of the pool isnot disturbed by the flow of material thereinto from the melting lowertip of the bar 32. The maintenance of a meniscus between the moltenlower end of the bar and the molten pool facilitates the addition ofmaterial without unduly disturbing the surface of the pool.

Another factor of significance in minimizing the effect of materialaddition on the thermal pattern of the pool is the location of theregion where the material is added. As may be seen in FIGURES 1 and 2,material addition is accomplished in a region outside of the annularregion of thermal turbulence in the pool (i.e., the impact area of theelectron beams). The thermal currents in the turbulent region act as adam to insure that the added material is melted and mixed before itenters the central region. Thus, it is possible in many instances to addparticulate material directly into the pool outside the turbulenceannulus without deleterious effect. Dopant for effecting thesemiconductor properties of the pulled crystal may also be added in thisway to insure thorough mixing.

To accurately control the position of the impact areas 14, a controlcircuit 40 (FIGURE 2) is provided suitably connected to each of thethree electron beam guns 22.

The control circuit operates to control the strengths of the fieldsproduced by the electromagnets 27 and hence the radius of curvature ofthe path which the electrons in the beam follow. By varying thestrengths of the fields produced by the electromagnets 27, therefore,the amount of deflection of the beams and hence the position of theimpact areas 14 may be varied. The control circuit is preferably soconstructed as to vary the strengths of the respective electromagnets 27in the electron guns 22 to make the positional variation of the impactareas 14 with respect to each other and to the axis of the singlecrystal 11 equal and symmetrical. By means of the latter technique, thethermal pattern in the molten pool 12 may be varied generallysymmetrically, as desired. A control circuit which may be adapted foruse in the system illustrated in FIGURES l and 2 is disclosed in US.Patent No. 3,235,647, assigned to the present assignee. A similarcontrol circuit may be adapted for use as the control circuit 38.

In addition to controlling the positions of the beam impact areas, thecontrol circuit 40 is also suitably con structed to control the power ofthe electron beams. This may be done by controlling the beam currentthrough variation in the temperature of the emitters in the guns, orthrough variation in the current output of the power supply for theguns. In either case, the amount of energy transferred to the pool bythe beams is controlled accordingly, facilitating the attainment ofconstant pool conditions (e.g., temperature and size).

Referring now to FIGURES 3 through 7, a specific manner of practicingthe invention will be described, although the method of the invention isnot limited to being practiced in this manner. FIGURES 3 through 7schematically illustrate various steps in using the apparatus of FIGURES1 and 2. For simplification, only one of the three beams is illustrated,but it is to be understood that all three beams are moved in the sameway. The crystal seed 25, held in the clamp 28, is lowered to withinabout one-quarter inch of the surface of the molten pool 12. Thisposition is illustrated in FIGURE 3. The positions of the beams aremoved in toward the axis of the seed 25 to a position A, the beams beingabout /8 to 1" from the seed. With the beams 13 being this close to theseed, the seed increases in temperature due to bombardment by fringeelectrons and an increased level of radiated heat. When the temperatureat the seed tip reaches approxi mately 900 C., the seed is dipped intothe pool to depth of about one sixteenth of an inch. This position isillustrated in FIGURE 4. At about the same time, the beams 13 are movedradially outward with respect to the axis of the seed, over a distanceof approximately one-quarter inch, from position A to a position Bindicated in FIGURE 4.

The rotating seed 25, because of the high temperature of the moltenpool, will melt slightly at the interface 31 and a meniscus 30 will format the surface of the molten pool 12. Once this meniscus is etsablishedbetween the rotating seed and the pool, the seed is drawn upwardly bythe connecting rod 29 (FIGURE 1) at a slow rate, for example, about twoinches per hour. At about the same time, the electron beams 13 are movedso that their impact areas 14 are at position C, illustrated in FIGURE5. At position C, the impact areas are approximately oneeighth of aninch closer to the axis of the seed than in position B, and theturbulence represented by the arrow 32 is also closer.

At the position C of the beam, illustrated in FIGURE 5, the surface ofthe single crystal growing from the seed 25 in the region 42 tapersinwardly at angle which is greater than 30 from the vertical due to thewashing action of the superheated melt in the turbulence region. Withthe growing crystal getting smaller in diameter than the seed at anangle of fresh surface greater than 30 from the vertical, crystalnucleation agents that may be residing on the surface of the seed growout of the crystal because the maximum angle at which they can grow isabout 30. Such crystal nucleation agents may consist of coating flakes,condensate on the seed surface (which are all amorphous atoms) and dirtfrom faulty seed handling procedures. In addition to minimizing theeffect of crystal nucleation agents residing on the surface of the seed,the growth condition maintained in the region 42 wipes out any influenceof disturbing conditions at the original surface of the seed. Thus, thesurface of the single crystal 11 being grown is of the sub-surfacequality of the seed.

The growth condition indicated in FIGURE 5' at region 42 will continueuntil the diameter of the growing crystal 11 is about one-eighth of aninch smaller than the diameter of the seed 25. After about a one-eighthof an inch reduction in diameter, a constant diameter growth will begin.As soon as constant diameter growth begins, the positions of the impactareas 14 of the beams 13 are moved radially outward about one-eighth tothree-eigths of an inch to position D illustrated in FIGURE 6. Thisreduces the washing action of the superheated melt and causes thegrowing crystal 11 to increase in diameter at a surface angle of about20, such as is indicated in FIG- URE 6 at the region 43.

As soon as the growth condition indicated at the region 43 begins, therate of movement of the connecting rod 29 (FIGURE 1) is increasedslowly, for example over a period of about 5 minutes, to increase thewithdrawal rate to about four or five inches per hour. This causes thegrowth angle of the crystal 11 to change to about 40 from the verticalindicated in the region 44 of FIGURE 6. Growth of the single crystal 11under the conditions indicated in the region 44 is continued until thecrystal reaches the nominal desired diameter. The proper selection ofthe beam position D will result in the crystal slowly changing thegrowth angle to to enable the maintenance of a constant desired diameterduring the remaining and major portion of the crystal growth. Some minorinward adjustment in beam position may be necessary in order to causethe crystal to turn the corner and change to a constant diameter growth.The constant diameter growth situation is indicated in the region 46 ofFIGURE 7.

In growing single silicon crystals, successful results may be achievedin utilizing the method of the invention for the following examples:

EXAMPLE 1 Seed crystal diameter: 0.200 inch Ultimate silicon crystaldiameter: 1.50 inches Furnace vacuum: 2X10 torr Average electron beampower: 16 kw.

Pool diameter: 8 inches Pool depth: 1 inch average Pool volume: approx.350 cc.

Seed crystal temperature at immersion: 900 C. Average surfacetemperature: 1435 C. Maximum surface temperature: approx. 1600 C.Initial withdrawal rate: 2 inches per hr.

Ultimate withdrawal rate: 6 inches per hr.

Rate of crystal rotation: 15 r.p.m.

Rate of silicon feed: 390 grams per hr.

Feed stock cut-off beam power: 2.1 kw.

Rate of crucible rotation: r.p.m. (opposite to crystal) EXAMPLE 2 Seedcrystal diameter: 4.5 mm.

Ultimate silicon crystal diameter: 20 mm. Furnace vacuum: 4 10 torrAverage electron beam power: 4.2 kw.

Pool diameter: 90 mm.

Pool depth: mm. avg.

Pool volume: approx. 75 cc.

Seed crystal temperature at immersion: 600 C. Average surfacetemperature: 1435 C. Maximum surface temperature: 1625 C. approx.Initial withdrawal rate: 5 cm./hr.

Ultimate withdrawal rate: 15 cm./ hr.

Rate of crystal rotation: r.p.m.

Rate of silicon feed: 5 mm./hr.

Feed stock cut-off beam power: 1.5 kw.

By way of further illustration, the method of the invention may bepractised for materials other than silicon as follows:

EXAMPLE 3 Material: B1900 nickel base alloy Seed crystal diameter: 4inch Ultimate grown crystal diameter: 2%. inches Furnace vacuum: 8X10torr Average electron beam power: 23 kw.

Pool diameter: 8 inches Pool depth: 4 inch avg.

Seed crystal temperature at immersion: 300 C. Average surfacetemperature: 1550 C. approx. Maximum surface temperature: 1800 C.approx.

Initial withdrawal rate: 6 inches/hr. Ultimate withdrawal rate: 12inches/ hr.

Since super alloy crystals or grains are usually of satisfactory qualitywhen grown in the naturally preferred direction of crystal growth (asopposed to silicon crystals for semiconductors), crystal diameter mayusually be increased directly to its ultimate value by moving the beamimpact area outwardly an appropriate distance once growth is begun.

Referring now to FIGURE 8, an alternative embodiment of apparatus forperforming the method of the invention is illustrated. Growth of asingle crystal 11 from the molten pool 13 in the crucible 18 and skull21, is accomplished by utilizing the clamp 28 and the connecting rod 29in accordance with the method as previously described. In the embodimentof FIGURE 8, however, the electron beam gun 47 is of generally annularconfiguration.

The gun 47 includes a ring-shaped cathode 48 and an annular shapingelectrode 49 having a cross section generally similar to that of theshaping electrode 26 illustrated in FIGURE 1. The accelerating anode 51is also of annular configuration and has a cross sectional shapegenerally similar to that of the anode 21 in FIGURE 1. A ring-shapedelectromagnet 52 with multiple windings of different average diametersis utilized to achieve variation in the mean diameter of the annularelectron beam 53 produced by the gun 47. The resulting impact pattern orarea 54 is of corresponding annular shape.

Thus, it will be appreciated that by varying the strength of the fieldestablished by the electromagnet 52 and by varying the effectivediameter of the coil windings of the electromagnet by suitable controlcircuits (not illustrated), the mean diameter of the impact area 54 maybe varied to produce corresponding variation in the thermal pattern ofthe pool 12. This variation may be utilized to achieve growth of asingle crystal as described in connection with FIGURES 3 through 7.Regulation of the beam power level may be utilized, as in the case ofthe guns 22, to maintain constant molten pool conditions.

Referring now to FIGURE 9, a further alternative embodiment of apparatusfor performing the method of the invention is illustrated. Growth of asingle crystal 11 from the molten pool 13 is accomplished by utilizingthe clamp 28 and the connecting rod 29 in accordance with the method aspreviously described. The apparatus is disposed within a vacuumenclosure, not illustrated, and the molten pool 11 is formed at the topof a vertical cylindrical pedestal of polycrystalline feed material,such as silicon. The pedestal 61 is preferably substantially greater indiameter than the ultimate diameter of the single crystal, and is movedupwardly at a rate selected to replenish material removed from the poolas the crystal is pulled. The thermal pattern in the molten pool isregulated so that the pool is contained, at its periphery 62, by surfacetension, the periphery of the pool thereby being about the same diameteras that of the pedestal 61. Although a pedestal-type feeding arrangementis illustrated, it is to be understood that the molten pool may becontained in a cooled crucible as in the previously describedembodiments.

As was the case in connection with the embodiment of FIGURE 8, anannular electron beam gun 47 is provided. The parts of the electron beamgun of FIGURE 9 are identical with those of the gun of FIGURE 8 and havebeen given identical reference numbers. The electrons are projected tothe surface of the pool 13 to form an annular impact pattern 63 thereon,and the electrons comprising the beam define a beam which is generallyhollow and frustoconical in configuration.

1n the embodiment of FIGURE 9, the electron beam produced by the gun 47is focused and controlled to regulate both the sharpness and the meandiameter of the annular impact pattern 63 on the surface of the pool 13.

This is accomplished by means of a pair of electromagnetic coils 64 and66, and by a pair of control circuits 67 and 68 connected to the coils64 and 66, respectively. The general configuration of the apparatus issimilar to that shown and described in US. Patent No. 3,105,275 assignedto the assignee of the present invention, and the method of the presentinvention includes a novel procedure for utilizing such apparatus. Theelectromagnetic coil 64 includes an annular core 69 of a material havinglow magnetic reluctance and is surrounded by electrically conductivewindings 71. The construction of the electromagnetic coil 66 is similarto that of the coil 64, including an annular low reluctance core 72surrounded by windings 73. The windings 71 of the electromagnetic coil64 are energized by the control circuit 67 which controls the magnitudeof the current flowing therethrough. Similarly, the windings 73 of theelectromagnetic coil 66 are energized by the control circuit 68, whichcontrols the magnitude of the electrical current flowing through thewindings 73.

The two electromagnetic coils 64 and 66 operate as a compound lens and,by way of analogy to glass lenses for focusing light rays, operate tochange the effective focal length of the combination lens by changingthe index of refraction of each lens. In terms of magnetic lenses, themagnetic induction of each electromagnetic coil is changed byappropriate changes in the current flowing therethrough. Byappropriately regulating the induction of each of the electromagneticcoils 64 and 66, the effective magnification of the combination lens canbe changed to thereby regulate the mean diameter of the annular beamimpact pattern 63 on the surface of the pool 13. The regulation of theinduction is selected in appropriate combinations to maintain sharpimaging of the beam on the surface and thereby provide a very narrowwidth of beam impact pattern, approximating the thickness of thefilament or cathode 48. By appropriate regulation of the two lenses, awide range of variation in the mean diameter of the impact pattern maybe achieved while maintaining sharpness in the image of the emitter(i.e., the impact pattern width).

By making the mean diameter of one or more of the electromagnetic lensessmaller, the image size, that is, the width of the annular impactpattern 63, may be made even smaller than the thickness of the emitter48. Thus, a plurality of concentric coils may be provided in place ofone or more of the electromagnetic coils 64 and 66, and each of theconcentric coils may be selectively energized for a desired image size.

Using an electron beam gun having an emitter mean diameter of 7", spaced6%" above the horizontal midplane of the coil 64, and 14%" above thesurface of the pool 13, wherein the inner diameter of the coil 64 is 11"and the inner diameter of the coil 66 is 6 /2", and wherein the top ofthe coil 66 is spaced 1" below the surface of the pool 13, the followingcombination of magnetic induction of the coils 64 and 66 give thefollowing listed mean diameter of the impact pattern 63 while producinga sharp image of the emitter 48 on the pool surface:

Mean

diameter Coil 64, C011 66, impact ampere ampere pattern, turns turnsinches 6.6)(10 4.s 1o 1% X10 10.5 3 4.6X10 1.1X10 3% 4X10 1.3)(10 42.4X10- 1.6X10 5 1.5)(10 1.7X10 6% 1.75X10 6 It may therefore be seenthat the invention provides an improved method for producing a singlecrystal. Crystal growth conditions are readily controlled by the methodof the invention, and the effect of nucleation agents residing on thesurface of the seed is minimized. In accordance with the invention, asmooth surface single crystal of high quality is produced, the crosssectional size of which may be selected as desired. Replenishment of themolten pool from which the single crystal is grown is readilyaccomplished without deleterious effect on growth conditions.

Various modifications of the invention, in addition to those shown anddescribed herein, will be apparent to those skilled in the art from theforegoing description and accompanying drawings. Such othermodifications are intended to fall within the scope of the appendedclaims.

What is claimed is:

1. A method of growing a single crystal from a molten pool of a materialfrom which a crystal is to be made comprising,

(a) contacting the surface of the molten pool with a single crystal seedto establish an interface between the seed and the molten pool,

(b) directing an electron beam onto the surface of the molten pool tocreate a region of turbulent flow of molten material within the moltenpool,

(c) withdrawing the seed from the molten pool at a rate such that asingle crystal of material is grown,

(d) and controlling the location of the impact area of the electron beamon the surface of the molten pool to control the distance of the regionof turbulent flow from the interface between the crystal and the moltenpool, thereby controlling the diameter of the single crystal beinggrown.

2. A method according to claim 1 wherein the electron beam is controlledso that the electron beam impact area is generally annular in shape andsubstantially surrounds the single crystal so that the region ofturbulent flow is substantially annular and surrounds the singlecrystal, whereby an inward flow of superheated molten material isestablished substantially surrounding the single crystal.

3. A method according to claim 2 including adding material to the moltenpool outside of the annular impact area.

4. A method according to claim 2 including adding a dopant to the moltenpool outside of the annular impact area.

5. A method according to claim 1 wherein the power of the electron beamis varied to control the average temperature and size of the moltenpool.

6. A method according to claim 2 wherein the electron beam is controlledso that the annular beam impact area is caused to move closer to theseed to cause a necking down of the single crystal at a surface anglegreater than 30 from the axis of the single crystal.

7. A method according to claim 6 wherein, after necking down, theelectron beam is controlled so that the annular beam impact is caused tomove farther from the seed to produce an enlargement of the singlecrystal cross section at a surface angle of about 20 from the axis ofthe single crystal.

8. A method according to claim 7, including substantially increasing therate of withdrawal after enlargement of the single crystal cross sectionbegins.

9. A method according to claim 8, wherein the increase in the rate ofWithdrawal is approximately doubled over a period of about 5 minutes.

10. A method according to claim 2, wherein the seed is positionedadjacent the pool surface, and wherein the electron beam is controlledto heat the seed by bombardment of fringe electrons and radiated heat.

11. A method according to claim 2, wherein the mean diameter of theannular beam impact area, and the width thereof, is controlled bycontrolling the current through a pair of annular electronl agneticlenses spaced axially 3,232,745 2/1966 Ru iiel et al. 148-1.6 along thepath of the electron beam. 3,278,274 10/ 1966 Liebmann et a1.

References Cited L. DEWAYNE RUTLEDGE, Primary Examiner UNITED STATESPATENTS 5 T. R. FRYE, Assistant Examiner 2,683,676 7/1954 Little et a11481.5 U.S. Cl. X.R.

2,858,199 10/1958 Larson 148--1.6 23-273, 301

UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 3 494804 Da February 10, 1970 lnventofls) Charles W. Hanks; Charles d'A. HuntIt is certified that error appears in the above-identified patent andthat said Letters Patent are hereby corrected as shown below:

Column 10, line 58, after "impact" insert -area.

SIGNED KND SEALED JUL141970 Edward M. Fletcher, 12:. m1: x. 60mm, JR-Anmin Office:

Comissioner of Patents

